Sagan-Mayr debate
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Can SETI Succeed:<br />
Carl <strong>Sagan</strong> and Ernst <strong>Mayr</strong> Debate<br />
With the development of technology and our present understanding of the laws of nature,<br />
humanity is now in a position to verify or falsify the belief in extraterrestrial civilizations using<br />
experimental test. SETI is the quest for a generally acceptable cosmic context for humankind. In<br />
the deepest sense, this search is a search for ourselves.<br />
Since the seminal paper by Giussepe Cocconi and Philip Morrison in 1959, the "orthodox view"<br />
among SETI proponents has been the following:<br />
Life is a natural consequence of physical laws acting in appropriate<br />
environments, and this physical process sequence — as took place on<br />
Earth — could occur elsewhere.<br />
SETI proponents argue that our own galaxy has hundreds of billions of<br />
stars, and we live in a universe with billions of galaxies, so life should<br />
be common in this cosmic realm. There should be many habitable<br />
planets, each sheltering its brood of living creatures. Some of these<br />
worlds should develop intelligence and the technological ability and<br />
interest in communicating with other intelligent creatures.<br />
Using electromagnetic waves, it should be possible to establish<br />
contact across interstellar distances and exchange information and wisdom around the galaxy.<br />
Some fraction of the extraterrestrial civilization should be providing an electromagnetic signature<br />
that we should be able to recognize.<br />
But because we have been unable to find a single piece of concrete evidence of alien intelligence<br />
yet, a philosophical battle has risen between those who might be called contact optimists — who<br />
generally embrace the orthodox view of SETI — and the proponents of the uniqueness<br />
hypothesis, which suggests that Earth is probably the only technological civilization in our galaxy.<br />
Here we present both sides of the philosophical and scientific <strong>debate</strong>. First, one of the most<br />
prominent evolutionary specialists of this century, Ernst <strong>Mayr</strong> of Harvard University's Museum of<br />
Comparative Zoology, delivers the main arguments of the uniqueness hypothesis. <strong>Mayr</strong> notes that,<br />
since they are based on facts, the various degrees of uniqueness are a problem for SETI, not a<br />
hypothesis. The late Carl <strong>Sagan</strong> of The Planetary Society and Cornell University's Laboratory of<br />
Planetary Studies responds to <strong>Mayr</strong>'s statements and expresses the optimist's view.<br />
Which view is more palatable? Read on and decide for yourself.<br />
The SETI <strong>debate</strong> originally appeared in the Planetary Society's Bioastronomy News, beginning<br />
with vol. 7, no. 3, 1995.)<br />
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Read Can SETI Succeed? Not Likely, by Ernst <strong>Mayr</strong><br />
Read The Abundance of Life-Bearing Planets, by Carl <strong>Sagan</strong><br />
Read Ernst <strong>Mayr</strong>'s final comments<br />
Read Carl <strong>Sagan</strong>'s final comments<br />
Return to SETI home<br />
The Uniqueness Hypothesis:<br />
Can SETI Succeed?<br />
Not Likely<br />
by Ernst <strong>Mayr</strong><br />
The View of<br />
the Optimists:<br />
The Abundance<br />
of Life-Bearing Planets,<br />
by Carl <strong>Sagan</strong><br />
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Can SETI Succeed? Not Likely<br />
By Ernst <strong>Mayr</strong><br />
What is the chance of success in the search for extraterrestrial intelligence? The answer to this<br />
question depends on a series of probabilities. I have attempted to make a detailed analysis of this<br />
problem in a German publication (<strong>Mayr</strong> 1992) and shall attempt here to present in English the<br />
essential findings of this investigation. My methodology consists in asking a series of questions<br />
that narrow down the probability of success.<br />
How Probable Is It That Life Exists Somewhere Else in the Universe?<br />
Even most skeptics of the SETI project will answer this question optimistically. Molecules that are<br />
necessary for the origin of life, such as amino acids and nucleic acids, have been identified in<br />
cosmic dust, together with other macromolecules, and so it would seem quite conceivable that life<br />
could originate elsewhere in the universe.<br />
Some of the modern scenarios of the origin of life start out with even simpler molecules--a<br />
beginning that makes an independent origin of life even more probable. Such an independent<br />
origin of life, however, would presumably result in living entities that are drastically different from<br />
life on Earth.<br />
Where Can One Expect To Find Such Life?<br />
Obviously, only on planets. Even though we have up to now secure knowledge only of the nine<br />
planets of our solar system, there is no reason to doubt that in all galaxies there must be millions if<br />
not billions of planets. The exact figure, for instance, for our own galaxy can only be guessed.<br />
How Many of These Planets Would Have Been Suitable for the Origin of Life?<br />
There are evidently rather narrow constraints for the possibility of the origin and maintenance of<br />
life on a planet. There has to be a favorable average temperature; the seasonal variation should<br />
not be too extreme; the planet must have a suitable distance from its sun; it must have the<br />
appropriate mass so that its gravity can hold an atmosphere; this atmosphere must have the right<br />
chemical composition to support early life; it must have the necessary consistency to protect the<br />
new life against ultraviolet and other harmful radiations; and there must be water on such a planet.<br />
In other words, all environmental conditions must be suitable for the origin and maintenance of life.<br />
One of the nine planets of our solar system had the right kind of mixture of these factors. This,<br />
surely, was a matter of chance. What fraction of planets in other solar systems will have an equally<br />
suitable combination of environmental factors? Would it be one in 10, or one in 100, or one in<br />
1,000,000? Which figure you choose depends on your optimism. It is always difficult to extrapolate<br />
from a single instance. This figure, however, is of some importance when you are dealing with the<br />
limited number of planets that can be reached by any of the SETI projects.<br />
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What Percentage of Planets on Which Life Has Originated Will Produce Intelligent Life?<br />
Physicists, on the whole, will give a different answer to this question than biologists. Physicists still<br />
tend to think more deterministically than biologists. They tend to say, if life has originated<br />
somewhere, it will also develop intelligence in due time. The biologist, on the other hand, is<br />
impressed by the improbability of such a development.<br />
Life originated on Earth about 3.8 billion years ago, but high intelligence did not develop until<br />
about half a million years ago. If Earth had been temporarily cooled down or heated up too much<br />
during these 3.8 billion years, intelligence would have never originated.<br />
When answering this question, one must be aware of the fact that evolution never moves on a<br />
straight line toward an objective ("intelligence") as happens during a chemical process or as a<br />
result of a law of physics. Evolutionary pathways are highly complex and resemble more a tree<br />
with all of its branches and twigs.<br />
After the origin of life, that is, 3.8 billion years ago, life on Earth consisted for 2 billion years only of<br />
simple prokaryotes, cells without an organized nucleus. These bacteria and their relatives<br />
developed surely 50 to 100 different (some perhaps very different) lineages, but, in this<br />
enormously long time, none of them led to intelligence. Owing to an astonishing, unique event that<br />
is even today only partially explained, about 1,800 million years ago the first eukaryote originated,<br />
a creature with a well organized nucleus and the other characteristics of "higher" organisms. From<br />
the rich world of the protists (consisting of only a single cell) there eventually originated three<br />
groups of multicellular organisms: fungi, plants and animals. But none of the millions of species of<br />
fungi and plants was able to produce intelligence.<br />
The animals (Metazoa) branched out in the Precambrian and Cambrian time periods to about 60<br />
to 80 lineages (phyla). Only a single one of them, that of the chordates, led eventually to genuine<br />
intelligence. The chordates are an old and well diversified group, but only one of its numerous<br />
lineages, that of the vertebrates, eventually produced intelligence. Among the vertebrates, a whole<br />
series of groups evolved--types of fishes, amphibians, reptiles, birds and mammals. Again only a<br />
single lineage, that of the mammals, led to high intelligence. The mammals had a long<br />
evolutionary history which began in the Triassic Period, more than 200 million years ago, but only<br />
in the latter part of the Tertiary Period-- that is, some 15 to 20 million years ago--did higher<br />
intelligence originate in one of the circa 24 orders of mammals.<br />
The elaboration of the brain of the hominids began less than 3 million years ago, and that of the<br />
cortex of Homo sapiens occurred only about 300,000 years ago. Nothing demonstrates the<br />
improbability of the origin of high intelligence better than the millions of phyletic lineages that failed<br />
to achieve it.<br />
How many species have existed since the origin of life? This figure is as much a matter of<br />
speculation as the number of planets in our galaxy. But if there are 30 million living species, and if<br />
the average life expectancy of a species is about 100,000 years, then one can postulate that there<br />
have been billions, perhaps as many as 50 billion species since the origin of life. Only one of these<br />
achieved the kind of intelligence needed to establish a civilization.<br />
To provide exact figures is difficult because the range of variation both in the origination of species<br />
and in their life expectancy is so enormous. The widespread, populous species of long geological<br />
duration (millions of years), usually encountered by the paleontologist, are probably exceptional<br />
rather than typical.<br />
Why Is High Intelligence So Rare?<br />
Adaptations that are favored by selection, such as eyes or bioluminescence, originate in evolution<br />
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scores of times independently. High intelligence has originated only once, in human beings. I can<br />
think of only two possible reasons for this rarity. One is that high intelligence is not at all favored by<br />
natural selection, contrary to what we would expect. In fact, all the other kinds of living organisms,<br />
millions of species, get along fine without high intelligence.<br />
The other possible reason for the rarity of intelligence is that it is extraordinarily difficult to acquire.<br />
Some grade of intelligence is found only among warm-blooded animals (birds and mammals), not<br />
surprisingly so because brains have extremely high energy requirements. But it is still a very big<br />
step from "some intelligence" to "high intelligence."<br />
The hominid lineage separated from the chimpanzee lineage about 5 million years ago, but the big<br />
brain of modern man was acquired less than 300,000 years ago. As one scientist has suggested<br />
(Stanley 1992), it required complete emancipation from arboreal life to make the arms of the<br />
mothers available to carry the helpless babies during the final stages of brain growth. Thus, a<br />
large brain, permitting high intelligence, developed in less than the last 6 percent of the life on the<br />
hominid line. It seems that it requires a complex combination of rare, favorable circumstances to<br />
produce high intelligence (<strong>Mayr</strong> 1994).<br />
How Much Intelligence Is Necessary To Produce a Civilization?<br />
As stated, rudiments of intelligence are found already among birds (ravens, parrots) and among<br />
non-hominid mammals (carnivores, porpoises, monkeys, apes and so forth), but none of these<br />
instances of intelligence has been sufficient to found a civilization.<br />
Is Every Civilization Able To Send Signals into Space and To Receive Them?<br />
The answer quite clearly is no. In the last 10,000 years there have been at least 20 civilizations on<br />
Earth, from the Indus, the Sumerian, and other near Eastern civilizations, to Egypt, Greece, and<br />
the whole series of European civilizations, to the Mayas, Aztecs, and Incas, and to the various<br />
Chinese and Indian civilizations. Only one of these reached a level of technology that has enabled<br />
them to send signals into space and to receive them.<br />
Would the Sense Organs of Extraterrestrial Beings Be Adapted To Receive Our Electronic<br />
Signals?<br />
This is by no means certain. Even on Earth many groups of animals are specialized for olfactory or<br />
other chemical stimuli and would not react to electronic signals. Neither plants nor fungi are able to<br />
receive electronic signals. Even if there were higher organisms on some planet, it would be rather<br />
improbable that they would have developed the same sense organs that we have.<br />
How Long Is a Civilization Able To Receive Signals?<br />
All civilizations have only a short duration. I will try to emphasize the importance of this point by<br />
telling a little fable.<br />
Let us assume that there were really intelligent beings on another planet in our galaxy. A billion<br />
years ago their astronomers discovered Earth and reached the conclusion that this planet might<br />
have the proper conditions to produce intelligence. To test this, they sent signals to Earth for a<br />
billion years without ever getting an answer. Finally, in the year 1800 (of our calendar) they<br />
decided they would send signals only for another 100 years. By the year 1900, no answer had<br />
been received, so they concluded that surely there was no intelligent life on Earth.<br />
This shows that even if there were thousands of civilizations in the universe, the probability of a<br />
successful communication would be extremely slight because of the short duration of the "open<br />
window."<br />
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One must not forget that the range of SETI systems is very limited, reaching only part of our<br />
galaxy. The fact that there are a near infinite number of additional galaxies in the universe is<br />
irrelevant as far as SETI projects are concerned.<br />
Conclusions: An Improbability of Astronomic Dimensions<br />
What conclusions must we draw from these considerations? No less than six of the eight<br />
conditions to be met for SETI success are highly improbable. When one multiplies these six<br />
improbabilities with each other, one reaches an improbability of astronomic dimensions.<br />
Why are there nevertheless still proponents of SETI?<br />
When one looks at their qualifications, one finds that they are almost exclusively astronomers,<br />
physicists and engineers. They are simply unaware of the fact that the success of any SETI effort<br />
is not a matter of physical laws and engineering capabilities but essentially a matter of biological<br />
and sociological factors. These, quite obviously, have been entirely left out of the calculations of<br />
the possible success of any SETI project.<br />
Now read <strong>Sagan</strong>'s response to <strong>Mayr</strong>'s critique<br />
Return to Making Contact<br />
Return to SETI home<br />
This article originally appeared in Bioastronomy News, vol. 7, no. 3, 1995.)<br />
One of the leading biologists of this century, Ernst <strong>Mayr</strong> is the Alexander Agassiz Professor of Zoology, Emeritus, of Harvard<br />
University and author of about 650 papers and 20 books. He is known for his work in ornithology and systematics and, as a leader<br />
in evolutionary biology, he has written about the development of species, over- population, biodiversity, and, most recently, SETI.<br />
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The Abundance of Life-Bearing Planets<br />
By Carl <strong>Sagan</strong><br />
We live in an age of remarkable exploration and discovery. Fully half of the nearby Sun-like stars<br />
have circumstellar disks of gas and dust like the solar nebula out of which our planets formed 4.6<br />
billion years ago. By a most unexpected technique -- radio timing residuals -- we have discovered<br />
two Earth-like planets around the pulsar B1257+12. An apparent Jovian planet has been<br />
astrometrically detected around the star 51 Pegasi.<br />
A range of new Earth-based and space-borne techniques--including astrometry,<br />
spectrophotometry, radial velocity measurements, adaptive optics and interferometry-- all seem to<br />
be on the verge of being able to detect Jovian- type planets, if they exist, around the nearest stars.<br />
At least one proposal (The FRESIP [Frequency of Earth Sized Inner Planets] Project, a<br />
spaceborne spectrophotometric system) holds the promise of detecting terrestrial planets more<br />
readily than Jovian ones. If there is not a sudden cutoff in support, we are likely entering a golden<br />
age in the study of the planets of other stars in the Milky Way galaxy.<br />
Once you have found another planet of Earth-like mass, however, it of course does not follow that<br />
it is an Earth- like world. Consider Venus. But there are means by which, even from the vantage<br />
point of Earth, we can investigate this question. We can look for the spectral signature of enough<br />
water to be consistent with oceans. We can look for oxygen and ozone in the planet's atmosphere.<br />
We can seek molecules like methane, in such wild thermodynamic disequilibrium with the oxygen<br />
that it can only be produced by life. (In fact, all of these tests for life were successfully performed<br />
by the Galileo spacecraft in its close approaches to Earth in 1990 and 1992 as it wended its way to<br />
Jupiter [<strong>Sagan</strong> et al., 1993].)<br />
The best current estimates of the number and spacing of Earth-mass planets in newly forming<br />
planetary systems (as George Wetherill reported at the first international conference on<br />
circumstellar habitable zones [Doyle, 1995]) combined with the best current estimates of the<br />
long-term stability of oceans on a variety of planets (as James Kasting reported at that same<br />
meeting [Doyle, 1995]) suggest one to two blue worlds around every Sun-like star. Stars much<br />
more massive than the Sun are comparatively rare and age quickly. Stars comparatively less<br />
massive than the Sun are expected to have Earth-like planets, but the planets that are warm<br />
enough for life are probably tidally locked so that one side always faces the local sun. However,<br />
winds may redistribute heat from one hemisphere to another on such worlds, and there has been<br />
very little work on their potential habitability.<br />
Nevertheless, the bulk of the current evidence suggests a vast number of planets distributed<br />
through the Milky Way with abundant liquid water stable over lifetimes of billions of years. Some<br />
will be suitable for life--our kind of carbon and water life--for billions of years less than Earth, some<br />
for billions of years more. And, of course, the Milky Way is one of an enormous number, perhaps a<br />
hundred billion, other galaxies.<br />
Need Intelligence Evolve on an Inhabited World?<br />
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We know from lunar cratering statistics, calibrated by returned Apollo samples, that Earth was<br />
under hellish bombardment by small and large worlds from space until around 4 billion years ago.<br />
This pummeling was sufficiently severe to drive entire atmospheres and oceans into space.<br />
Earlier, the entire crust of Earth was a magma ocean. Clearly, this was no breeding ground for life.<br />
Yet, shortly thereafter--<strong>Mayr</strong> adopts the number 3.8 billion years ago--some early organisms arose<br />
(according to the fossil evidence). Presumably the origin of life had to have occupied some time<br />
before that. As soon as conditions were favorable, life began amazingly fast on our planet. I have<br />
used this fact (<strong>Sagan</strong>, 1974) to argue that the origin of life must be a highly probable<br />
circumstance; as soon as conditions permit, up it pops!<br />
Now, I recognize that this is at best a plausibility argument and little more than an extrapolation<br />
from a single example. But we are data constrained; it's the best we can do.<br />
Does a similar analysis apply to the evolution of intelligence?<br />
Here you have a planet burgeoning with life, profoundly changing the physical environment,<br />
generating an oxygen atmosphere 2 billion years ago, going through the elegant diversification<br />
that <strong>Mayr</strong> briefly summarized-- and not for almost 4 billion years does anything remotely<br />
resembling a technical civilization emerge.<br />
In the early days of such <strong>debate</strong>s (for example, G.G. Simpson's "The Non-prevalence of<br />
Humanoids") writers argued that an enormous number of individually unlikely steps were required<br />
to produce something very like a human being, a "humanoid"; that the chances of such a precise<br />
repetition occurring on another planet were nil; and therefore that the chance of extraterrestrial<br />
intelligence was nil. But clearly when we're talking about extraterrestrial intelligence, we are not<br />
talking--despite Star Trek--of humans or humanoids. We are talking about the functional<br />
equivalent of humans-- say, any creatures able to build and operate radio telescopes. They may<br />
live on the land or in the sea or air. They may have unimaginable chemistries, shapes, sizes,<br />
colors, appendages and opinions. We are not requiring that they follow the particular route that led<br />
to the evolution of humans. There may be many different evolutionary pathways, each unlikely, but<br />
the sum of the number of pathways to intelligence may nevertheless be quite substantial.<br />
In <strong>Mayr</strong>'s current presentation, there is still an echo of "the non-prevalence of humanoids." But the<br />
basic argument is, I think, acceptable to all of us. Evolution is opportunistic and not foresighted. It<br />
does not "plan" to develop intelligent life a few billion years into the future. It responds to<br />
short-term contingencies. And yet, other things being equal, it is better to be smart than to be<br />
stupid, and an overall trend toward intelligence can be perceived in the fossil record. On some<br />
worlds, the selection pressure for intelligence may be higher; on others, lower.<br />
If we consider the statistics of one, our own case--and take a typical time from the origin of a<br />
planetary system to the development of a technical civilization to be 4.6 billion years--what<br />
follows? We would not expect civilizations on different worlds to evolve in lock step. Some would<br />
reach technical intelligence more quickly, some more slowly, and-- doubtless--some never. But the<br />
Milky Way is filled with second- and third-generation stars (that is, those with heavy elements) as<br />
old as 10 billion years.<br />
So let's imagine two curves: The first is the probable timescale to the evolution of technical<br />
intelligence. It starts out very low; by a few billion years it may have a noticeable value; by 5 billion<br />
years, it's something like 50 percent; by 10 billion years, maybe it's approaching 100 percent. The<br />
second curve is the ages of Sun-like stars, some of which are very young-- they're being born right<br />
now--some of which are as old as the Sun, some of which are 10 billion years old.<br />
If we convolve these two curves, we find there's a chance of technical civilizations on planets of<br />
stars of many different ages--not much in the very young ones, more and more for the older ones.<br />
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The most likely case is that we will hear from a civilization considerably more advanced than ours.<br />
For each of those technical civilizations, there have been tens of billions or more other species.<br />
The number of unlikely events that had to be concatenated to evolve the technical species is<br />
enormous, and perhaps there are members of each of those species who pride themselves on<br />
being uniquely intelligent in all the universe.<br />
Need Civilizations Develop the Technology for SETI?<br />
It is perfectly possible to imagine civilizations of poets or (perhaps) Bronze Age warriors who never<br />
stumble on James Clerk Maxwell's equations and radio receivers. But they are removed by natural<br />
selection. The Earth is surrounded by a population of asteroids and comets, such that occasionally<br />
the planet is struck by one large enough to do substantial damage. The most famous is the K-T<br />
event (the massive near- Earth-object impact that occurred at the end of the Cretaceous period<br />
and start of the Tertiary) of 65 million years ago that extinguished the dinosaurs and most other<br />
species of life on Earth. But the chance is something like one in 2,000 that a civilization-destroying<br />
impact will occur in the next century.<br />
It is already clear that we need elaborate means for detecting and tracking near-Earth objects and<br />
the means for their interception and destruction. If we fail to do so, we will simply be destroyed.<br />
The Indus Valley, Sumerian, Egyptian, Greek and other civilizations did not have to face this crisis<br />
because they did not live long enough. Any long- lived civilization, terrestrial or extraterrestrial,<br />
must come to grips with this hazard. Other solar systems will have greater or lesse asteroidal and<br />
cometary fluxes, but in almost all cases the dangers should be substantial.<br />
Radiotelemetry, radar monitoring of asteroids, and the entire concept of the electromagnetic<br />
spectrum is part and parcel of any early technology needed to deal with such a threat. Thus, any<br />
long-lived civilization will be forced by natural selection to develop the technology of SETI. (And<br />
there is no need to have sense organs that "see" in the radio region. Physics is enough.)<br />
Since perturbation and collision in the asteroid and comet belts is perpetual, the asteroid and<br />
comet threat is likewise perpetual, and there is no time when the technology can be retired. Also,<br />
SETI itself is a small fraction of the cost of dealing with the asteroid and comet threat.<br />
(Incidentally, it is by no means true that SETI is "very limited, reaching only part of our galaxy." If<br />
there were sufficiently powerful transmitters, we could use SETI to explore distant galaxies;<br />
because the most likely transmitters are ancient, we can expect them to be powerful. This is one<br />
of the strategies of the Megachannel Extraterrestrial Assay [META].)<br />
Is SETI a Fantasy of Physical Scientists?<br />
<strong>Mayr</strong> has repeatedly suggested that proponents of SETI are almost exclusively physical scientists<br />
and that biologists know better. Since the relevant technologies involve the physical sciences, it is<br />
reasonable that astronomers, physicists and engineers play a leading role in SETI.<br />
But in 1982, when I put together a petition published in Science urging the scientific respectability<br />
of SETI, I had no difficulty getting a range of distinguished biologists and biochemists to sign,<br />
including David Baltimore, Melvin Calvin, Francis Crick, Manfred Eigen, Thomas Eisner, Stephen<br />
Jay Gould, Matthew Meselson, Linus Pauling, David Raup, and E.O. Wilson. In my early<br />
speculations on these matters, I was much encouraged by the strong support from my mentor in<br />
biology, H.J. Muller, a Nobel laureate in genetics. The petition proposed that, instead of arguing<br />
the issue, we look:<br />
We are unanimous in our conviction that the only significant test of the existence of extraterrestrial<br />
intelligence is an experimental one. No a priori arguments on this subject can be compelling or<br />
should be used as a substitute for an observational program.<br />
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Now Read Ernst <strong>Mayr</strong>'s response<br />
Read Carl <strong>Sagan</strong>'s final comments<br />
Return to Making Contact<br />
Return to SETI home<br />
This article originally appeared in the Planetary Society's Bioastronomy News, vol. 7, no. 4, 1995.)<br />
One of the founding members of the Planetary Society and a major scientific figure until his death in 1996, Carl <strong>Sagan</strong> was a major<br />
supporter of SETI, promoting its possibilities through his writing. Read more about Carl <strong>Sagan</strong>'s scientific contributions and<br />
achievements.<br />
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The SETI Debate:<br />
Ernst <strong>Mayr</strong> Responds<br />
Response to "The Abundance of Life-Bearing Planets"<br />
I fully appreciate that the nature of our subject permits only<br />
probabilistic estimates. There is no argument between Carl <strong>Sagan</strong> and<br />
myself as to the probability of life elsewhere in the universe and the<br />
existence of large numbers of planets in our and other nearby<br />
galaxies. The issue, as correctly emphasized by <strong>Sagan</strong>, is the<br />
probability of the evolution of high intelligence and an electronic<br />
civilization on an inhabited world.<br />
Once we have life (and almost surely it will be very different from life<br />
on Earth), what is the probability of its developing a lineage with high<br />
intelligence? On Earth, among millions of lineages of organisms and<br />
perhaps 50 billion speciation events, only one led to high intelligence;<br />
this makes me believe in its utter improbability.<br />
<strong>Sagan</strong> adopts the principle "it is better to be smart than to be stupid," but life on Earth refutes this<br />
claim. Among all the forms of life, neither the prokaryotes nor protists, fungi or plants has evolved<br />
smartness, as it should have if it were "better." In the 28 plus phyla of animals, intelligence evolved<br />
in only one (chordates) and doubtfully also in the cephalopods. And in the thousands of<br />
subdivisions of the chordates, high intelligence developed in only one, the primates, and even<br />
there only in one small subdivision. So much for the putative inevitability of the development of<br />
high intelligence because "it is better to be smart."<br />
<strong>Sagan</strong> applies physicalist thinking to this problem. He constructs two linear curves, both based on<br />
strictly deterministic thinking. Such thinking is often quite legitimate for physical phenomena, but is<br />
quite inappropriate for evolutionary events or social processes such as the origin of civilizations.<br />
The argument that extraterrestrials, if belonging to a long-lived civilization, will be forced by<br />
selection to develop an electronic know-how to meet the peril of asteroid impacts is totally<br />
unrealistic. How would the survivors of earlier impacts be selected to develop the electronic<br />
know-how? Also, the case of Earth shows how impossible the origin of any civilization is unless<br />
high intelligence develops first. Earth furthermore shows that civilizations inevitably are short-lived.<br />
It is only a matter of common sense that the existence of extraterrestrial intelligence cannot be<br />
established by a priori arguments. But this does not justify SETI projects, since it can be shown<br />
that the success of an observational program is so totally improbable that it can, for all practical<br />
purposes, be considered zero.<br />
All in all, I do not have the impression that <strong>Sagan</strong>'s rebuttal has weakened in any way the force of<br />
my arguments.<br />
Ernst <strong>Mayr</strong><br />
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Carl <strong>Sagan</strong> Responds<br />
Is Earth-Life Relevant? A Rebuttal<br />
The gist of Professor <strong>Mayr</strong>'s argument is essentially to run through the<br />
various factors in the Drake equation (see Shklovskii and <strong>Sagan</strong>,<br />
1966) and attach qualitative values to each. He and I agree that the<br />
probabilities concerning the abundance of planets and the origins of<br />
life are likely to be high. (I stress again that the latest results [Doyle,<br />
1995] suggest one or even two Earth-like planets with abundant<br />
surface liquid water in each planetary system. The conclusion is of<br />
course highly tentative, but it encourages optimism.) Where <strong>Mayr</strong> and<br />
I disagree is in the later factors in the Drake equation, especially those<br />
concerning the likelihood of the evolution of intelligence and technical<br />
civilizations.<br />
<strong>Mayr</strong> argues that prokaryotes and protista have not "evolved<br />
smartness." Despite the great respect in which I hold Professor <strong>Mayr</strong>, I<br />
must demur: Prokaryotes and protista are our ancestors. They have evolved smartness, along<br />
with most of the rest of the gorgeous diversity of life on Earth.<br />
On the one hand, when he notes the small fraction of species that have technological intelligence,<br />
<strong>Mayr</strong> argues for the relevance of life on Earth to the problem of extraterrestrial intelligence. But on<br />
the other hand, he neglects the example of life on Earth when he ignores the fact that intelligence<br />
has arisen here when our planet has another five billion years more evolution ahead of it. If it were<br />
legitimate to extrapolate from the one example of planetary life we have before us, it would follow<br />
that:<br />
● There are enormous numbers of Earth-like planets, each stocked with enormous numbers<br />
of species, and<br />
● In much less than the stellar evolutionary lifetime of each planetary system, at least one of<br />
those species will develop high intelligence and technology<br />
Alternatively, we could argue that it is improper to extrapolate from a single example. But then<br />
<strong>Mayr</strong>'s one-in-50 billion argument collapses. It seems to me he cannot have it both ways.<br />
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On the evolution of technology, I note that chimpanzees and bonobos have culture and<br />
technology. They not only use tools but also purposely manufacture them for future use (see<br />
<strong>Sagan</strong> and Druyan, 1992). In fact, the bonobo Kanzi has discovered how to manufacture stone<br />
tools.<br />
It is true, as <strong>Mayr</strong> notes, that of the major human civilizations, only one has developed radio<br />
technology. But this says almost nothing about the probability of a human civilization developing<br />
such technology. That civilization with radio telescopes has also been at the forefront of weapons<br />
technology. If, for example, western European civilization had not utterly destroyed Aztec<br />
civilization, would the Aztecs eventually--in centuries or millennia--have developed radio<br />
telescopes? They already had a superior astronomical calendar to that of the conquistadores.<br />
Slightly more capable species and civilizations may be able to eliminate the competition. But this<br />
does not mean that the competition would not eventually have developed comparable capabilities<br />
if they had been left alone.<br />
<strong>Mayr</strong> asserts that plants do not receive "electronic" signals. By this I assume he means<br />
"electromagnetic" signals. But plants do. Their fundamental existence depends on receiving<br />
electromagnetic radiation from the Sun. Photosynthesis and phototropism can be found not only in<br />
the simplest plants but also in protista.<br />
All stars emit visible light, and Sun-like stars emit most of their electromagnetic radiation in the<br />
visible part of the spectrum. Sensing light is a much more effective way of understanding the<br />
environment at some distance; certainly much more powerful than olfactory cues. It's hard to<br />
imagine a competent technical civilization that does not devote major attention to its primary<br />
means of probing the outside world. Even if they were mainly to use visible, ultraviolet or infrared<br />
light, the physics is exactly the same for radio waves; the difference is merely a matter of<br />
wavelength.<br />
I do not insist that the above arguments are compelling, but neither are the contrary ones. We<br />
have not witnessed the evolution of biospheres on a wide range of planets. We have not observed<br />
many cases of what is possible and what is not. Until we have had such an experience--or<br />
detected extraterrestrial intelligence--we will of course be enveloped in uncertainty.<br />
The notion that we can, by a priori arguments, exclude the possibility of intelligent life on the<br />
possible planets of the 400 billion stars in the Milky Way has to my ears an odd ring. It reminds me<br />
of the long series of human conceits that held us to be at the center of the universe, or different<br />
not just in degree but in kind from the rest of life on Earth, or even contended that the universe<br />
was made for our benefit (<strong>Sagan</strong>, 1994). Beginning with Copernicus, every one of these conceits<br />
has been shown to be without merit.<br />
In the case of extraterrestrial intelligence, let us admit our ignorance, put aside a priori arguments,<br />
and use the technology we are fortunate enough to have developed to try and actually find out the<br />
answer. That is, I think, what Charles Darwin--who was converted from orthodox religion to<br />
evolutionary biology by the weight of observational evidence--would have advocated.<br />
Carl <strong>Sagan</strong><br />
Return to Making Contact<br />
Return to SETI home<br />
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